The tact switch is used to input an electrical signal or switch an electrical power between the terminals connected to a metal dome by arranging a conductive elastic contact or an elastic metal (referred to as ‘metal dome’) on a board having a pattern terminal and then by pressing and releasing the metal dome.
The tact switch is used for a keypad board such as a MP3 player, a MP4 player (video player), a mobile telecommunication terminal, a PDA, etc. or a print circuit board (PCB). The tact switch repeats either of the connection and the disconnection of an electrical signal or a power whenever a user pushes and releases the tact switch.
As mobile telecommunication and data processing devices such as a mobile telecommunication terminal, a PDA, etc. or a print circuit board are miniaturized and light-weighted, the tact switch is also required to be miniaturized.
Conventional tact switch has a size of about 4 mm×4 mm×0.5 mm (width×length×thickness). The tact switch is being made by a terminal-pattern PCB is prepared by consecutively printing switch terminals having the same pattern on epoxy board or phenol board (mostly, epoxy board is used), coating insulation material on portions except the terminal pattern, arranging a contact above the terminal pattern, covering with a dust cover, and cutting each PCB having the contact in a contact unit.
There is a rise in demand for the ultra-small tact switch having the minimized size of 2 mm×3 mm×0.5 mm (width×length×thickness) and due to the competition to reduce the thickness of the mobile devices, even the difference of a 0.1 mm width is required and thus, the reduction of the width of the tact switch even by the order of 0.1 mm is very important to reduce the thickness and weight of the mobile device.
FIG. 1 shows a cross section of a conventional tact switch 10. As shown in FIG. 1, the tact switch 10 comprises an insulation film 11, a print circuit board 12, a base 13, a metal dome 14 and a top coverlay 16. The top coverlay 16 is arranged on the base 13 such that it covers an inner space of the base 13 which receives the metal dome 13 therein.
If the size of the tact switch 10 becomes smaller, the size of the tact switch 10 depends on an expected bonding width T0 of the bonding area of the top coverlay 16, thereby influencing waterproofness. That is, as shown in FIG. 1, if the outer rim of the top coverlay 16 is inaccurately in contact with the outer rim of the base 13, the outer rim of the top coverlay 16 is spaced apart from the outer rim of the base 13 the by the distance of T2. In this case, the bonding width T1 where the top coverlay 16 is in contact with the base 13 becomes smaller than the expected bonding width T0 by the distance of T2 and thus, the bonding width T1 with regard to the top surface of the base 13 is smaller than the expected bonding width T0, thereby resulting in a poor bonding of the top coverlay 16.
When the top coverlay 16 is incompletely bonded to the base 13, the repetition of use of the tact switch 10 causes the incomplete bonding area of the top coverlay 16 with regard to the base 13 to be lifted.
If the tact switch 10 is repeatedly used, the conventional top coverlay 16 cannot perfectly seal the inner space of the based 13 on which the metal dome 14 rests and thus, the print circuit board 12 cannot be waterproof when water comes into electrical devices having the tact switch 10.
If moisture comes into the tact switch 10, a pop-up occurs such that an element located between the tact switch 10 and the print circuit board 12 becomes unfastened, thereby generating a deformation or crack of the element and decreasing a moisture sensitivity level (MSL).
It was very difficult to fasten the top coverlay 16 to the base 13 such that the arrangement of the outer rim of the top coverlay 16 to correspond to the outer rim of the base is made to a tolerance of 0.1 mm.
Further, as the tact switch 10 becomes smaller, the metal dome 14 also becomes smaller. If the size of the metal dome is reduced, the repetition of use of the tact switch 10 damages the metal dome 14, thereby reducing the lifetime of the metal dome 14. This impairs the reliability of the tact switch and in turn, restricts the reduction of the metal dome size.
Tact switch for mobile phone keypads is disclosed in Korean patent laid-open publication no. 10-2005-0014359.